Synchronous Optical NETworking (SONET) is an American National Standards Institute (ANSI) standard for synchronous data transmission on optical media. An equivalent international standard to SONET is called synchronous digital hierarchy (SDH). Although the following discussion revolves around SONET, it should be apparent to a person skilled in the art where parallels may be drawn between the two standards.
In a SONET based transmission system, signals are arranged into “Synchronous Transport Signal Level 1 (STS-1)” signals with a basic bit rate of 51.84 Mbps. SONET includes a set of signal rate multiples for transmitting digital signals on optical fiber. The base rate, called Optical Carrier 1 and typically referred to as OC-1, is 51.84 Mbps. An STS-1 signal is carried on a link with an OC-1 signal rate, called an OC-1 link. An OC-2 link runs at twice the base rate (103.68 Mbps) and carries an STS-2 signal. An OC-3 link runs at three times the base rate (155.52 Mbps) and carries an STS-3 signal. Higher rates include OC-12 (622.08 Mbps), OC-48 (2.488 Gbps) and OC-192 (9.95328 Gbps). For simplicity, OC-48 and OC-192 links are often considered to run at 2.5 Gbps and 10 Gbps respectively.
A standard STS-1 frame consists of 6480 bits. The frame is divided into time slots containing 8-bit bundles, called octets or bytes, such that the frame may be seen to be organized in rows of bytes and columns of time slots. For STS-1 specifically, the frame may be considered as nine rows and 90 columns. Not all of an STS-1 frame is payload, as 36 octets (the first three columns) are reserved for “transport overhead” (TOH), which is used for such purposes as frame identification and monitoring of errors. The other 87 columns comprise a synchronous payload capacity. Into the capacity is mapped an 87 column synchronous payload envelope (SPE). Typically, the SPE consists of one column of path overhead and 86 columns of payload.
The transport overhead is comprised of three rows of “section overhead” and six rows of “line overhead”. While formulating an STS-1 frame, a local network element generates the line overhead based on a supplied frame capacity. The line overhead and the frame capacity is then amalgamated and scrambled, for instance, to balance the outgoing signal to ensure that an even distribution of ones and zeros are transmitted. The local network element then generates section overhead based on the scrambled line overhead and frame capacity amalgamation.
In the section overhead, one byte, designated “B1”, is used for error monitoring. The first bit of B1 is set such that the total number of ones in the first positions of all octets in the previous STS-1 frame, after scrambling, is always an even number. The second bit of B1 is used in the same way, in respect of the second bit of each octet in the STS-1 frame. The remaining bits follow this pattern.
Additionally, in the line overhead, one byte, designated “B2”, is used for error monitoring. Where B1 is set relative to the previous STS-1 frame after scrambling, each bit of B2 is set relative to each correspondingly positioned bit in the amalgamated line overhead and frame capacity of the previous STS-1 frame before scrambling.
Typically, a local network element generates a signal of multiple STS-1 frames and sends the signal to a remote network element on an OC-N link. The remote network element receives the signal and extracts information from the transport overhead. A new transport overhead is generated for the signal before the signal is sent on to a third network element. In particular, with respect to error monitoring bytes B1 and B2, the same computing procedures used to create B1 and B2 are performed on a given received STS-1 frame. The results are compared to a B1 and B2 extracted from the STS-1 frame following the given received STS-1 frame. A count of the number of discrepancies is reported by the remote network element to a network management system. The count for B1 indicates the number of differences between a received B1 and a calculated B1. This process of computing, comparing and reporting is called terminating.
Many other bytes are present in the line overhead, including S1, Z1, H1 and H2. S1 is defined in the SONET standard as: Synchronization Status (S1)—The S1 byte is located in the first STS-1 frame of an STS-N frame, and bits 5 through 8 of that byte are allocated to convey the synchronization status of the network element through a Synchronization Status Message (SSM). This SSM is a four-bit code used to indicate synchronization status. Bits 1 through 4 of the S1 byte are currently undefined. The Z1 byte is called a growth byte and takes the same position in the line overhead as the S1 byte, but only in the STS-1 frames other than the first STS-1 frame of an STS-N frame. The H1 and H2 bytes are called pointer bytes and provide an indication of the offset between the pointer bytes and the beginning of the SPE. Synchronization Status Message Definitions are presented in Table 1.
TABLE 1Synchronization Status Message DefinitionsQualityS1 bitsDescriptionAcronymLevel5678Stratum 1 TraceablePRS10001Synchronized -STU20000Traceability UnknownStratum 2 TraceableST230111Transit Node ClockTNC40100TraceableStratum 3E TraceableST3E51101Stratum 3 TraceableST361010SONET Minimum ClockSMC71100TraceableStratum 4 TraceableST48N/ADON'T USE forDUS91111SynchronizationProvisionable byPNOuserthe Network Operatorassignable
Where several separate low-speed SONET signals are to be sent in one high-speed SONET signal, there is a need for a combiner. A combiner serves the purpose, at a sending end, of arranging data from a number of signals into a single, complex signal. At the receiving end, the single signal is divided out into the separate signals by a demultiplexer.
Many STS-1 frames may be multiplexed (combined) into a single STS-N frame. In the STS-N frame, many of the transport overhead bytes only have meaning in the first STS-1 frame. The section overhead (including byte B1) of the first STS-1 frame in an STS-N frame is used to carry information about the entire STS-N frame. However, the line overhead (including byte B2) of each STS-1 frame in the STS-N frame is maintained. A maximum B1 count that may be reported at the receiving end of an exemplary OC-48 link is eight (per STS-48 frame). However, at the receiving end of the same link, the maximum B2 count is 384, which is eight per STS-1 frame for 48 frames.
In an exemplary 4:1 SONET multiplexing application, four STS-48 signals (on OC-48 links) are combined to give one STS-192 signal (on an OC-192 link). The combiner terminates the B1 and B2 bytes of the input STS-48 signals. Since B1 and B2 are terminated at the STS-48 signal input, numbers of errors in each OC-48 link are determined and reported to the network management system by the combiner. At the remote end of the OC-192 link, the four STS-48 signals may be extracted from the received STS-192 signal by a demultiplexer and sent, individually, to four intended remote network elements at the ends of four separate OC-48 links.
When the B1 and B2 bytes are terminated at the remote network elements, the error information represented by the resultant counts will only reflect errors on the OC-48 link from the demultiplexer to the remote network element. Furthermore, the S1 byte in each STS-48 frame will hold an SSM code relating to the synchronization status of the demultiplexer.
An STS-48 frame in the above example traverses three links, namely two OC-48 links and an OC-192 link. It is desirable that the multiplexing process be transparent to the remote network element, i.e., that the three links are considered as a single OC-48 link. However, error information regarding the first OC-48 link and synchronization status of the local network element are typically unavailable to the remote network element.